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Inheritance of haemophilia

haemophilia is usually inherited, meaning that it is passed from parent to child through the parent’s genes. Genes are packaged within the body’s cells on structures called chromosomes. The genes involved in haemophilia are located on the "X" chromosome.

The X chromosome is also called the "sex chromosome" because it plays a role in determining whether a person is male or female. Men have one X chromosome, which they inherit from their mother, and one Y chromosome, which they inherit from their father. Women have two X chromosomes: one from each parent.

If the X chromosome a man inherits from his mother has the altered or changed gene, he will have haemophilia. If a woman inherits a copy of the altered gene from either of her parents, she is said to "carry" the haemophilia gene and is therefore called a "carrier". In other words, she has one normal and one altered copy of the gene.

with haemophilia
Mother carrying 
haemophilia gene

All daughters of a father with haemophilia will inherit his altered copy of the X chromosome.

Carriers have one normal and one altered gene. They have a 50% chance of passing the altered gene on to each child. Boys who inherit the altered gene from their mother will have haemophilia; girls who inherit the altered gene from their mother are carriers.

Lyonization and clotting factor levels

In each cell in a woman’s body, one of the two X chromosomes is turned off, or "suppressed". This process is called "lyonization", after Mary Lyon, who first described it. Lyonization is a random process that is not fully understood.

If the chromosome that’s turned off has the altered gene, that cell will produce clotting factor. If the chromosome with the normal gene is turned off, the cell will not produce clotting factor, or the clotting factor it makes won’t work properly.

On average, carriers of haemophilia will have about 50 per cent of the normal amount of clotting factor, because about half of their cells will have the "good" gene turned off. Some carriers have far lower levels of clotting factor because more of the X chromosomes with the normal gene are switched off.